脉冲磁场下电磁力特性研究:理论分析

利用数学解析方法对脉冲磁场细晶技术的电磁力特性进行了理论分析.分析结果显示,单个放电周期内,随着时间变化,熔体内部脉冲电磁力方向发生瞬时改变;在某个时间段范围内,熔体内部与外部存在电磁力方向不一致的现象,而且单个周期内电磁力存在衰减特性.以上这些特性,是造成脉冲磁场致熔体振荡的主要原因.另外,对于本研究采用的工频脉冲磁场,脉冲电磁力对熔体还有搅拌作用.而且,当脉冲放电频率与凝固系统的某一固有频率相符合时,将引起共振效应,增强脉冲电磁力作用效果.     

基于多重分形研究脉冲磁场周期对凝固组织的影响

为了研究脉冲磁场对凝固组织的影响,采用自发研制的脉冲磁场发生装置将脉冲磁场非接触式地施加到钛处理低碳钢的凝固过程中,利用金相显微镜和实验室设计的多重分形软件研究了不同周期的脉冲磁场对凝固组织的影响。结果表明,该钢种的凝固组织具有分形特征;随着脉冲磁场周期的延长,Δα和Δf(α)呈现为先减小后增大的规律,在脉冲磁场周期为0.5 s时,组织最为均匀。由此可以推断脉冲磁场周期对凝固组织的影响存在一个最佳值。     

低压脉冲磁场凝固硅钢的热变形组织和行为

为了细化硅钢铸锭组织，提高其加工性能，研究了低压脉冲磁场对硅钢凝固组织的影响规律，并对比了不同凝固组织的热变形行为和组织。研究结果表明，在硅钢的凝固过程中施加脉冲磁场，传统的柱状晶组织转变为等轴晶组织。随着磁场频率的增加，等轴晶比率(等轴晶面积/整体面积)先增加后降低，等轴晶晶粒尺寸先减小后增大。随着励磁电压增加，等轴晶比率增加，等轴晶晶粒尺寸不断增加，获得最佳脉冲磁场工艺参数，磁场频率为5 Hz,励磁电压为200 V。在该脉冲磁场工艺参数下，硅钢凝固组织中的等轴晶尺寸细化至无磁场作用时的1/2,等轴晶比率提高至100%。对无脉冲磁场的柱状晶组织和施加脉冲磁场(磁场频率为5 Hz,励磁电压为200 V)的等轴晶组织，进行了热压缩试验(温度为950℃,应变速率为0.01 s^-1和0.1 s^-1)和热轧试验(初轧温度1 100℃)。通过微观组织结果发现，无脉冲磁场作用的柱状晶凝固组织在热压缩过程中发生少量动态再结晶，组织中存在难以消除的变形晶粒，影响后续加工性能。脉冲磁场作用下等轴晶凝固组织在相同热压缩条件下获得细小均匀的再结晶组织，平均晶粒尺寸...     

脉冲磁场下电磁力特性研究:凝固实验

采用低熔点金属伍德合金研究了工频脉冲磁场对金属液面波动行为的影响规律:工频脉冲磁场的周向电磁力搅拌熔体使其形成环状涡流;而径向电磁力在其表现为压力作用时使熔体在中心形成圆形液柱,表现为拉力作用时使熔体在中心形成圆形漩涡.另外,采用Sn-20%Pb合金研究了工频脉冲磁场对铸坯质量的影响:工频脉冲磁场的作用不仪使Sn-Pb合金的微观组织明显细化,而且使铸坯的表面质量得到明显改善.在本实验条件下,当脉冲放电频率为3.9和6.6Hz时,金属熔体的液面发生共振运动,加速了熔体内的流动,从而增强了脉冲磁场的细化效果.     

氧化物冶金与脉冲磁场参数影响钢组织的研究

 脉冲磁场处理与氧化物冶金技术是细化组织、提升材料性能的两种常用方法,将其有机结合可进一步优化钢铁材料的性能。利用自主研制的高频感应线圈加热炉与脉冲磁场发生装置将脉冲磁场非接触式地施加在钛处理低碳钢的凝固过程中,利用金相显微镜、多重分形软件与维氏硬度仪研究了不同脉冲磁场参数对凝固组织的影响。结果表明,脉冲磁场感应强度为135~190 mT、磁场作用时间为5~10 min时,试样的金相组织最细小均匀,原始奥氏体晶粒得到明显细化,原始奥氏体晶粒面积由15.79 mm2下降到1.25 mm2,试样的硬度值由118.1HV提升到165.4HV,此参数下的脉冲磁场对凝固组织的细化程度最佳。     

脉冲磁场强度对钛处理低碳钢凝固过程的影响

采用自发研制的脉冲磁场发生装置将脉冲磁场非接触式地施加到钛处理低碳钢的凝固过程中,利用金相显微镜、扫描电镜和能谱仪研究了不同强度的脉冲磁场对试样的宏观形貌、枝晶和组织的影响。结果表明,随着脉冲磁场强度的增加,试样的宏观表面逐渐变得平整;枝晶和组织随着脉冲磁场强度的增加逐渐细化,但达到一定强度后,继续增加又出现了相反的结论。由此可以推断,脉冲磁场强度对枝晶和组织的影响存在一个最佳值。     

脉冲电磁场凝固组织细化和均质化技术研究与应用进展

由于金属凝固过程中选分结晶的作用,不可避免地会出现成分不均匀现象。连铸过程中由于强制冷却,这种成分不均匀现象更为严重。这不仅影响了铸坯和铸锭后续加工性能,并影响了最终产品质量和性能的均匀性和稳定性。研究和生产实践表明,细化凝固组织是解决成分不均匀性的有效手段。脉冲电磁场因能耗低、施加方便、细晶效果显著,近年来得到了广泛关注,有望成为冶金界广泛应用的凝固组织细化和均质化技术。分别介绍脉冲电流、脉冲磁场和脉冲磁致振荡3种脉冲电磁场凝固组织细化和均质化技术的研究现状和应用进展。     

有无电磁场作用下S45C钢的凝固组织

研究了S45C碳钢凝固时，同时接受垂直方向的静态磁场作用以及具有水平分量的交流电。因此，受到电磁力作用的试样，在凝固过程中受到电磁力的影响，将不同电磁搅拌条件下的凝固试样进行切割，然后用化学侵蚀以观察其宏观和显微组织，并且对它们进行了比较。没有实施静态磁场和交流电的钢中，其显微组织为枝晶结构。另一方面，在1特斯拉静态磁场和80A，2KHZ的交流电作用下，其凝固组织为等轴结构。当磁场强度减少到0．3T时，部分凝固组织为等轴结构。在磁场强度为1T常数下，改变交流电频率，使其数值低于80A，凝固组织从等轴结构变为枝晶结构。晶粒细化机理假定为通过电磁力不均匀的分布，对试样进行对流感应后，将枝晶打碎成碎片，使其转变成等轴结构，通过提高电流频率强化电磁力。     

小方坯凝固末端电磁搅拌的磁场数值模拟

 利用ANSYS软件对160 mm×160 mm小方坯凝固末端电磁搅拌器所产生的电磁场分布进行了模拟。计算了小方坯内部磁场分布情况,同时计算出在不同搅拌电流条件下的铸坯凝固末端凝固前沿钢液液芯所受的电磁力与钢液的转速。结果表明,铸坯凝固前沿磁感应强度、电磁力及钢液转速均随着搅拌电流的增加而增大。结合实际生产情况,确定凝固末端液芯半径为175 mm,电磁搅拌频率为6 Hz时,最佳搅拌电流为400 A,此状态下凝固前沿由电磁力引起的钢液最大转速为12 r/s。     

脉冲磁场对钛处理低碳钢凝固组织的影响

采用自行研制的脉冲磁场发生装置对含Ti低碳钢的凝固过程进行控制,利用金相显微镜、扫描电镜和能谱仪研究不同放电电压条件下的脉冲磁场对钢中夹杂物的析出行为及凝固组织的影响规律,分析不同凝固冷却速度对脉冲磁场细晶化效果的影响。结果表明:随着放电电压的升高,夹杂物的尺寸先减小后增大,晶内铁素体的数量逐渐增多,凝固组织变得更细小;降低凝固冷却速度,可延长脉冲磁场对钢液凝固过程的作用时间,晶粒的细化效果更明显;当放电电压为80 V,凝固冷却速度为60℃/min时,凝固组织几乎全部由细小的相互交错的针状铁素体组成。     

Structure Evolution and Solidification Behavior of Austenitic Stainless Steel in Pulsed Magnetic Field

 To understand the solidification behavior of austenitic stainless steel in pulsed magnetic field, the solidification process is investigated by means of the self made high voltage pulse power source and the solidification tester. The results show that the solidification structure of austenitic stainless steel can be remarkably refined in pulsed magnetic field, yet the grains become coarse again when the magnetic intensity is exceedingly large, indicating that an optimal intensity range existed for structure refinement. The solidification temperature can be enhanced with an increase in the magnetic intensity. The solidification time is shortened obviously, but the shortening degree is reduced with the increase of the magnetic intensity.     

Research on the grain refinement under pulsed magnetic field

In this paper,the principle of Pulsed Magnetic Field(PMF) force was analyzed through mathematical analyses.By theoretical analysis and calculation,the results show that the great electromagnetic force is made in the melt under pulsed magnetic field,as well as changing its direction in different places of melt at the same time.It enforces the crystallizing nucleus and brittle crystallite to fragment in the solidification processing.From the point of view,one of the main factors of grain refinement is that the fragmentations occur under a pulsed magnetic field by preliminary judgement. The feasibility of application in the metallurgical industry under PMF was discussed through comparing the results of grain refinement under EMS.According to the theoretic calculation,the power consumption under EMS is 5 -8 times the amount under PMF,when both of magnetic flux density B are 0.07T.That is to say,the better effect on grain refining can be obtained under PMF,compared with EMS,even in the lower power consumption.The solidification experiments of Sn-20%Pb alloy are conducted under the same experimental conditions that the magnetic intensity is 0.07T in the center of the crucible,it also shows that PMF has a better effect on grain refining than EMS. Combined with the continuous casting process,the influence of pulsed parameters and the metallurgical effects with applying PMF at different solidification stages was investigated.There are different grain refining effects under PMF in different solidification stages,and there are different grain refining effects under PMF in different PMF parameters.For the Sn-20%Pb alloy or silicon steel,it is more effective during the initial stage,in which the pulse frequency is 5Hz.For the Sn-20%Pb alloy,the average grain size ofβphase is the smallest,when applying the PMF during the temperature of melt decreasing from 201℃to 184℃.Further investigation of the specified technique parameters for industrial applications are required.     

Progress in Research on Solidification in a Strong Static Magnetic Field

Strong magnetic fields available from superconducting magnets are opening a way to new phenomena that could lead to new methods in materials processing including solidification. The principal research involving solidification in strong static magnetic fields is emphasizing four aspects: control of crystal orientation, convection damping, thermoelectric magnetohydrodynamics (TEMHD) and change in thermodynamics. Under high magnetic intensity, aligned structural textures are induced in both magnetic and non‐magnetic materials. Since in strong magnetic field the melt flow is suppressed by convection damping, the microstructure being formed during solidification is affected heavily; this phenomenon applies to eutectic, monotectic and peritectic alloys as well as to dendritic morphologies typical of directional solidification. If strength and orientation of a magnetic field are controlled appropriately, this strong damping effect will generate more homogeneous crystals as a result of achieving diffusion‐controlled solute transport conditions. TEMHD more easily occurs in strong magnetic fields, resulting in equiaxed crystals even under directional solidification. It is evidenced experimentally and theoretically that the thermodynamics of phase transformation and nucleation are changed by strong magnetic fields.     

Effects of Pulsed Current and Pulsed Magnetic Field Complex Pretreatment on Martensite Transformation of Cr5 Steel during Continuous Cooling

Carbide precipitation and martensite transformation in Cr5 steel have been observed in situ by high-temperature confocal laser-scanning microscopy.In this way,the influences of pulsed current and pulsed magnetic field complex pretreatment on carbide precipitation and martensite transformation during continuous cooling have been studied.The results show that the electropulsing complex pretreatment promotes the precipitation of M_7C_3-type carbides at high temperature,increases the start and finish temperatures of martensite transformation,and extends the phase transformation time.Martensite prefers to nucleate in the austenite with less precipitation of carbides due to the chemically homogeneous distribution of solute atoms.     

外磁场在定向凝固过程中的应用研究

以磁场对合金凝固组织的影响为基础,综述了近年来交变磁场和稳恒磁场在定向凝固过程中的应用和发展,总结了外磁场对定向凝固过程中合金微观组织的作用机制,并展望了今后的研究方向。